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N. Fukuta

Abstract

The fall velocities and growth rates of freely falling ice crystals were measured at various intervals for about 1 min after ice crystal nucleation of a supercooled fog. Measurements were also made at various degrees of supercooling.

The results showed that: 1) the velocity varied linearly with time t for 60 sec after the first 12-sec period, with two falling velocity plateaus being found at −5 and −8, and −10 and −18C; 2) the mass varied as t 1/2 except at −18 to −19C, a small mass growth rate peak being found at −6.5C; 3) the larger peak shifted toward colder regions as the crystal grew; 4− tsuzumi crystals formed at around −18C; and 5) at the temperatures where two maxima of the mass growth occurred, the apparent densities were at minima.

Current theory failed to describe the mass growth rate of ice crystals. The existence of an unknown factor was detected and introduced into the rate equation of ice crystal growth.

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N. Fukuta

Abstract

When a highly volatile liquid is sprayed into moist air the evaporation of the liquid droplets may cool the air sufficiently for the condensed water droplets to freeze by homogeneous nucleation. This paper describes the use of this technique to produce about 1010 ice crystals per gram of volatile liquid.

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N. Fukuta

Abstract

No abstract available.

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N. Fukuta

Abstract

The ability of 329 selected organic compounds to nucleate ice has been tested by three methods. Their activity was found to depend strongly on the method of preparation. More than 20 compounds were found to nucleate ice at temperatures above −5C when freshly ground. Some meta and para derivatives of benzene showed excellent activity. Particles prepared by condensation were less active, the only newly-discovered material effective above −5C being 1,5-dihydroxynaphthalene. Its use as a cloud seeding agent is suggested.

Crystals of effective materials generally have molecules which display rotational symmetry. The role of molecular symmetry is discussed in connection with the formation of crystal planes with dense hydrogen bonding groups. The behavior of organic nuclei with nonsymmetrical molecules is also considered.

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N. Fukuta

Abstract

Ice nucleation and subsequent phase equilibrium in water held in the micro-capillaries of atmospheric particles are examined. It is found that ice formed in the micro-capillaries of certain particles may exist in equilibrium with a dry atmosphere, where particles preactivated by Fournier's effect are expected to lose their activity. A possible mechanism of activation of the particles as ice nuclei in a cold dry atmosphere is suggested.

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N. Fukuta

Abstract

The ice-nucleating ability of certain organic materials may be further enhanced by spraying solutions of these materials in suitable organic solvents. By utilizing the evaporative cooling of the solvent, ice nucleation in clouds may be activated at temperatures approaching 0C. The factors governing the suitability of various solvents are discussed. It is found that immiscibility with water is a desirable property.

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N. Fukuta
and
Y. Paik

Abstract

Aerosols of organic ice nucleating agents were formed by rapid adiabatic expansion, by passing superheated steam laden with the organic vapors through a supersonic nozzle. The effect of the Mach number of the flow through the nozzle on the aerosol particle size and number concentration was investigated. It was found that the number of particles formed increased by a factor of 103 as the Mach number increased from 1 to 2, but further increases in Mach number had little effect. The ice nuclei activity spectra of the organic aerosols thus generated showed plateaus at colder temperatures, with approximately 1014 nuclei per gram below −9°C for 1,5-dihydroxynaphthalene and 1012 g−1 below −7°C for metaldehyde.

Coagulation rates of both organic smokes were determined. Coagulation of 1,5-dihydroxynaphthalene aerosols followed the Smoluchowski equation. Coagulation of metaldehyde aerosols was not only extremely rapid, but in addition the coagulation constant increased with time. The latter effect was attributed to the strong electrical dipole of metaldehyde particles.

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N. Fukuta
and
Y. Paik

Abstract

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M. N. Plooster
and
N. Fukuta

Abstract

This paper describes a numerical model of ice-phase precipitation from orographic clouds which includes the effects of seeding with artificial ice nuclei. The model describes the events which take place when a layer of moist air of near-neutral stability, overlain by a more stable dry layer, flows over a mountain ridge. A two-dimensional, steady-state model of the flow in the vertical plane normal to the ridge furnishes a field of the flow streamlines along which microphysical processes are followed. The cloud physics model describes the formation of the supercooled cloud, the formation of ice particles from both natural and artificial ice nuclei, and the growth and precipitation of ice particles. Growth by both vapor deposition and riming are included. Artificial ice nuclei are released from a localized source at ground level. A simple Fickian diffusion process is used to describe the vertical transport of nuclei to the cold upper region of the cloud.The model calculates the rate of precipitation at ground level as a function of horizontal position. The dependence of precipitation efficiency on cloud top temperature is in good agreement with field observations from the Climax experiment. Substantial precipitation increases are produced by seeding clouds whose natural precipitation efficiencies are low. However, it is shown that the seeding rate required to produce a given increase in precipitation rate is a strong function of cloud temperature. The model also shows that precipitation rates depend upon the activity of the ice nuclei as a function of temperature.

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N. Fukuta
and
V. K. Saxena

Abstract

A kinetic analysis of the fog formation process due to the contact of warm, moist air with a cold wall is presented. The analysis reveals that a wave in the nominal saturation ratio forms at and spreads away from the wall. The threshold conditions under which the maximum saturation ratio at the wave head becomes unity are derived. Comparison is made between the thermodynamic treatment of air mass mixing and the kinetic treatment. It is shown that the difference in diffusivities of vapor and temperature is responsible for the discrepancy of the two treatments. Results are applied to the problems of air sample processing, to avoid transient supersaturations, in cold chambers. The necessity of considering the kinetic effects of molecular diffusion in advection fog formation over the ocean is demonstrated.

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